The present invention relates generally to improved sealants, and more particularly to convenient, economical, and environmentally non-threatening compositions and methods used for certain specialty sealing and adhesive applications.
In recent years, there has been a great increase in the popularity of containerized, curable foam sealants used for a number of purposes. Urethane foams made from two separate components are effective and economical forms of sealants that are able to be generated at or near the site of application. Such foams comprise the reaction product of two separate components, one being a hydroxyl-rich resin and the other component containing an isocyanate component. A dissolved blowing agent is usually provided in one or both of the components. The combination of the exothermic heat of reaction and the reduction in pressure as the contents are released from a pressurized container into the atmosphere allows the blowing agent to vaporize, and this causes the resinous material to expand while it is crosslinking or curing.
One disadvantage of two-component systems is that the components must be thoroughly mixed immediately before discharge at or adjacent to the place of deposition. This requires relatively precise mixing apparatus, and while it is very advantageous to use such equipment in commercial and industrial applications, and in certain sorts of new construction, for example, where a great deal of attention can be paid to the conditions of application, such systems are not always preferred from the standpoint of convenience, reliability, and cost. These systems are rarely used by the do-it-yourself consumer.
Thus, the mixing and dispensing operations must be able to be carried on if the presence of a relatively accurate metering so that the resultant product is chemically and structurally satisfactory. Inasmuch as the two components react rapidly with each other, application must immediately follow mixing, and a disposable nozzle is required for each application sequence. Thus, once the components mix, they react and further dispensing cannot be achieved unless the sources of the components remain isolated from each other.
A consequence of the foregoing has been that for smaller scale applications, especially in sealing and adhesive applications and small-scale void filling, including the do-it-yourself or hobby market and the smaller scale portions of the residential or commercial markets, so-called single component foams have achieved a dominant position in the market. Single component foams are so-called because they dispense from a single container without the need for component mixing or nozzle replacement. Their constituents are actually two essential components premixed within a container, but the crosslinking or curing reaction does not take place until the contents have been discharged from the container, whereupon the curing reactions that result in a finished product take place.
In the last 10 years or more, the blowing agents necessary to transform solidifying liquid reactants into foam have been chosen from a small list of products. The commonly used agents for both two component and single component foams have been liquefiable gases, including hydrocarbons, hydrochlorof luorocarbons, hydrof luorocarbons, and chlorofluorocarbons. These chemicals usually serve as both partial propellants and blowing agents for the foam. One advantage of a liquefiable propellant is the inherently somewhat self-regulating nature of the achievable pressure, and the concomitant ability to dissolve or at least store significant amounts of blowing agent/propellant within a container that can tolerate only modest pressures.
In many prior art two component and single component urethane foams, the liquified gas creates a pressure in the headspace that acts as all or part of the propellant, but a much greater quantity is dissolved in the liquid constituent, serving as an expanding or blowing agent converting the solidifying liquid or gel-like reactants into a foam product, usually of a closed-cell, rigid type for purposes of sealing or the like.
Unfortunately, it has been discovered that the use of liquefiable gaseous blowing agents have substantial drawbacks. Many of the blowing agents/propellants formerly used were chlorofluorocarbons, which have been determined to be hazardous to the environment. Such materials, when escaping to the upper atmosphere and exposed to radiation, undergo chemical reactions resulting in a depletion of the concentration of ozone in the upper layers of the atmosphere. Accordingly, such blowing agents are no longer considered desirable or legal for such applications.
Pure hydrocarbons, such as butane, propane, or mixtures of these and other of their isomers also have the advantage of being readily liquified. However, the gaseous hydrocarbons can be a serious fire hazard. Especially in construction areas, it is not desired to leave flammable residues in the product, or create a flammable gas atmosphere in any of the work areas where the products are dispensed. Additionally, hydrocarbons create a Volatile Organic Compound (VOC) problem contributing to ground level smog.
In the past, and in fact, in the earliest use of urethanes, carbon dioxide (CO.sub.2) was used as a blowing agent for urethane foams. However, such CO.sub.2 was not supplied as a gas independently of the reaction, but resulted from reacting a portion of the isocyanate with water. The remaining portion was used to create part of the polymers. Historically speaking, it was common to provide hydroxyl-rich resins, and a small amount of water in part `B` which, when exposed to the isocyanate, in part `A`, would create the carbon dioxide blowing agent that resulted in the production of a polyurethane foam product.
The principal drawbacks of such chemistry were twofold. First, the system was practical only where two components could be kept separate until reaction. Second, the isocyanate is the most expensive constituent of polyurethane foams, and using it as a source of CO.sub.2 is not considered desirable. This is what created the impetus to search for other propellants in the past, and was essentially responsible for the development and widespread use of chlorofluorocarbons and similar liquefiable gases as foaming agents/propellants.
The present invention overcomes the disadvantages and drawbacks of the prior art by providing a novel and previously thought impossible foam frothing alternative for blowing or frothing single component foams using non-liquefiable gases. Historically, single component foams relied on liquefiable gases (as described above) which generally were thought required for both propellant and blowing agent. In this invention, it is demonstrated that non-liquefiable gases such as carbon dioxide (CO.sub.2) and nitrous oxide (N.sub.2 O) can be used to provide foams using conventional single component foam chemical raw materials.
Because the need for expansible sealants and adhesives which could be propelled from their containers and expanded by the use of inert, non-flammable, non-threatening materials has not been met, it is an object of the present invention to provide an improved foam type expansible sealant/adhesive propelled by an inert, non-environmentally threatening material.
A further object of the invention is to provide a sealant composition which may be dispensed under the pressure of an inert gas thereby propelling the sealant from a container and through a narrow dispensing tube to the point of application, which the inert gas will expand the composition sufficiently to provide an adhesive bead of an insulating resinous material.
A still further object of the invention is to provide a sealant with insulating and adhesive properties that is propelled from the container and expanded by means of carbon dioxide or other non-liquefiable gas initially present in the container.
Another object of the invention is to provide a so-called single component expansible adhesive sealant that will meet the current application requirements for this type of product and act as an adhesive in specific applications.
Yet another object of the invention is to provide a primarily CO.sub.2 -expanded urethane sealant which requires no premixing and in which the curing or crosslinking reaction is activated by discharge in a location having ambient atmospheric moisture, and which may include small proportions of supplemental propellants or the like.
Still another object of the invention is to provide a sealing and insulating cellular adhesive composition that is economical and easy to use.
An additional object of the invention is to provide an adhesive sealant composition having ingredients which permit dissolving of an inert, propellant/foaming gas in order to increase the amount of inert gas that can be stored within a container at pressures considerably less than the liquefiable pressure of CO.sub.2 and within the pressure limits of commercially available aerosol containers.
The invention achieves its objects and others by providing an adhesive/sealant, expanded by a non-liquefiable gas propelled from a container initially and expanded by a blowing agent consisting primarily or exclusively of a non-liquefiable gas and formulated from a moisture curable polyurethane prepolymer and reacted within the container using a low molecular weight isocyanate of two or more functionality, and in one or more variations, contains solubility enhancers for gaseous CO.sub.2 or supplemental propellants/liquefiable blowing agents.
The manner in which the invention achieves its objects and advantages will become more clearly apparent when reference is made to the following illustrative examples of the practice of the invention appearing below.